[CMake] Support generating Config.h

Summary:
This patch removes the hand maintained config files in favor of auto-generating the config file. We will still need to maintain the defines for the Xcode builds on Mac, but all CMake builds use the generated header instead.

This will enable finer grained platform support tests and enable supporting LLDB on more platforms with less manual maintenance.

I have only tested this patch on Darwin, and any help testing it out on other platforms would be greatly appreciated. I've probably messed something up somewhere.

Reviewers: labath, zturner

Reviewed By: labath

Subscribers: krytarowski, emaste, srhines, lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D31969

llvm-svn: 300372
diff --git a/lldb/source/Host/common/File.cpp b/lldb/source/Host/common/File.cpp
index c81ed5f..1869a6d 100644
--- a/lldb/source/Host/common/File.cpp
+++ b/lldb/source/Host/common/File.cpp
@@ -307,7 +307,7 @@
 
 Error File::GetFileSpec(FileSpec &file_spec) const {
   Error error;
-#ifdef LLDB_CONFIG_FCNTL_GETPATH_SUPPORTED
+#ifdef F_GETPATH
   if (IsValid()) {
     char path[PATH_MAX];
     if (::fcntl(GetDescriptor(), F_GETPATH, path) == -1)